Composition of copper small cents, aka green pennies

[Nick A]

I am curious as to why dug indian heads and early wheats come out of the ground with such nice green patina. It's too much for me to believe this is just chance that coins after the mid-1920's do not exhibit this patination.

I believe the answer lies in the "5% tin and zinc" content of bronze cents. I cannot find a reference to the specific percentages of zinc and tin used. Sometime in the early 1920s, the percentage of zinc must have been increased and tin decreased. I can find no reference to this change, but I would be very interested in finding one.

Tin is a key element in bronze. The properties of tin state that it is not easily oxidized, prevents corrosion, resists corrosion from water, but can be attacked by acids, alkalis and acid salts. These are exactly the properties that would make the early cents better preserved than the later ones.

Anyone who has dug an eaten up modern penny knows how zinc weathers in the ground. So, this also indicates that an increase in zinc would make the surfaces more easily corroded.

Thoughts? Evidence?

 

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[l.cutler]

The compostition of the cent was the same from late 1864 until 1962, except for the 1943-1945. Maybe just the length of time in the ground makes the difference?

 

[Nick A]

The composition is the "same" only to the extent that it is "5% tin and zinc" this is a vague reference. I can find no references that specify the specific percentages of tin and zinc.

My theory is the older "green" cents are maybe 3% tin and 2% zinc and the non-green are maybe 1% tin and 4% zinc.

It has little to do with the time in the ground, as these earlier cents even read differently on the meter of many metal detectors, which says to me that it is clear there is some difference in metallic content. The conductive value of older cents on the Minelab E-Trac is 33-36, the conductive value of later cents is 42-43. And this can be shown in the ground and in air tests with dug and non-dug coins, so the time in the ground is not significant.

 

[DEMERSON]

How about minerals in the ground

 

[Nick A]

Well, the minerals or ground effect only prevent the green patination from forming or eat it away. If the soil is "acid, alkali or contains acid salts" then the tin will be attacked and the coin will have a rough corroded surface.

Where the soil is fairly neutral, the tin provides some protection against corrosion. There is some oxidation/corrosion occurring in the ground as that is what makes the cent turn green.

My hypothesis is a greater amount of tin in the mix helps keep the surfaces smooth and even, preventing (in more neutral soils) the rough or heavy corrosion as seen in many copper coins recovered from fertilized farm fields.

I find that in my soil, which is fairly stable and neutral that older small cents tend to have this smooth patination. Coins that have been fertilized or are in clay type soil do not. Even modern coins in clay type soil are heavy corroded.

Overall I guess my question is not, "Why do they turn green?" but rather, why are the early cents so much better preserved than later cents when supposedly the metals are the same. I am saying is the metals must not really be the same, and the ratios of tin to zinc within that 5% have been altered.

Another random thought is wondering if the addition of tin to the copper coating on modern zinc pennies would improve their corrosion resistance. Hmm...

 

[mts]

Interesting thread. I have no additional info for you. But I think you are onto something when you point out that these different pennies read differently on your metal detector. If they had the exact same content you'd expect them to read very similarly.

 

[Nick A]

Brass, Bronze?

Wikipedia shed some light on some things:

Brass is any alloy of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. In comparison, bronze is principally an alloy of copper and tin.

Aluminium makes brass stronger and more corrosion resistant. Aluminium also causes a highly beneficial hard layer of aluminium oxide (Al2O3) to be formed on the surface that is thin, transparent and self healing. Tin has a similar effect and finds its use especially in sea water applications (naval brasses). Combinations of iron, aluminium, silicon and manganese make brass wear and tear resistant. A well known alloy used in the automotive industry is 'LDM C673', where the combination of manganese and silicon leads to a strong and resistant brass.

Selective leaching, also called dealloying, demetalification, parting and selective corrosion, is a corrosion type in some solid solution alloys, when in suitable conditions a component of the alloys is preferentially leached from the material. The less noble metal is removed from the alloy by microscopic-scale galvanic corrosion mechanism. The most susceptible alloys are the ones containing metals with high distance between each other in the galvanic series, eg. copper and zinc in brass.